1. Field of Invention
The present invention is directed to a dryer system and dryer structure to pre-dry for example "wet" fuels, such as for example bagasse, prior to it being used as a combustible source of heat energy in for example a waste fuel boiler in the sugar cane processing industry, the preferred embodiment using the boiler stack gases as the drying medium in a vertical dryer utilizing uniform, evenly distributed, non-vortex flow of the drying gases and the "wet" fuel to be dried.
2. Prior Art
It is old broadly to pre-dry fuels usually with a mechanical dryer. A typical fuel dryer structure in the prior art of fuel dryers generally comprises a horizontal drum dryer which is gas or oil fired and in which the whole dryer body or drum rotates.
The present invention, it is believed, has especially advantageous application to the field of waste fuel boilers such as are found in the sugar cane processing industry wherein bagasse, the residue of sugar cane, is used as a primary source of fuel.
A 1948 article entitled "Utilizing Bagasse as Fuel" discussed the use of bagasse as a fuel. The article pointed out that the low efficiency in bagasse boiler operation is mainly the result of the large amount of water vapor present in the combustion space. The loss factor is 20% to 25% of the total boiler energy. About two-thirds of this effect derives from the moisture content of the bagasse, and one-third from the products of combustion.
Partial prior art remedies have included preheating of combustion air, feedwater heating, revamping of combustion arrangements and enlarging furnace volume. Results range from good to marginal. When designed into the most modern bagasse boilers, the combination of these features might give a boiler the capability of running on bagasse alone. Even so, the capacity of such a boiler is reduced when running on bagasse alone, primarily because the furnace is doubling as a dryer.
From an engineering standpoint this amounts to treating the symptoms while ignoring the root cause of the malaise. Engineering studies of bagasse drying date back more than half a century. But, the increased capacity and efficiency, and the cleaner burning of partially dried bagasse were apparently outweighed by the availability of cheap supplementary fuels (gas, oil), and the former sufficiency of steam generated.
Recently the sugar mills have had to face a succession of interacting impacts:
1. The need for more boiler output, to supply the increased energy needs within the mill; PA1 2. The mandatory cleanup of stack effluents; and PA1 3. The increasing unavailability and the escalating cost of natural gas and fuel oil. PA1 1. Maximum drying effect through evenly distributed, controlled cross flow of hot gases and bagasse, both on the conical surfaces and in the openings in between. PA1 2. Automatic tendency to prevent choking. If overfeed of bagasse chokes dryer, as soon as the feed rate is reduced to normal, the dryer automatically begins to unclog, due to the higher blower backpressure and the action of the wipers. PA1 3. Practically eliminates fire hazard. Concurrent flow directions mixes hottest gases with wettest bagasse at upper end, and vice versa at lower end. Automatic hot gas diverter (dampered gas bypass) can be used to isolate dryer and smother fire if it should ever occur. PA1 4. Bagasse retention time in dryer and final degree of dryness can be regulated over a wide range of fuel types and conditions and boiler combustion condition by using the proper settings of the adjustable speed drive, the adjustable wipers, the gas flow to the dryer, and the selection of the slope angles and spacing of the fixed and rotating materials trays. PA1 5. Rotating shaft and cones are removable from top of dryer without dismantling or disconnecting ducts. Makes easy repair or replacement of wear surfaces. PA1 6. Low power consumption for effect achieved. Power of rotating shaft and cones is low, and non-surging due to even distribution of materials within dryer. Advantageous aerodynamic flow patterns minimize power consumption of fan forcing gases through dryer. PA1 7. Due to the evenness of distribution of both gases and solids flowing through the dryer body, a positive filtering effect occurs due to the crossflow of solids and gases at each solids dropoff location. Additional separation of solids from the gas stream also occurs at each level where a centrifugal effect occurs due to the gases changing direction. Thus, the gases are automatically subjected to a continuous cleaning effect. This is important where a substantial polluting effluent is typical, as for example, with a boiler burning waste fuel. PA1 8. The use of this design dryer in conjunction with a waste fuel boiler, where the dried fuel is fed to the boiler, will enable increased boiler-furnace combustion rate, boiler efficiency and steam output, and, by virtue of a cleaner burning effect within the furnace, reduce substantially the initial airborne solids and gaseous effluent (per unit of fuel) leaving the boiler.
The author's conclusion in the above referenced 1948 article, re the economic feasibility of drying bagasse fuel, was prophetic.